1. Field of the Invention
The present invention relates to a steam turbine power plant and more particularly to a digital electrohydraulic (DEH) turbine control system which incorporates an improved speed monitoring system for generation of an actual turbine speed measurement signal for controlling turbine speed and load.
2. Prior Art Discussion
A DEH turbine control system, presently in use, uses a programmed digital computer to output position setpoints to servo loops associated with control of the steam inlet valves to accelerate the turbine from turning gear to line frequency, and to control the load output of the turbine once the turbine power plant has been coupled to the power system network. To effectively protect and control the turbine through a startup and while on-line, process variables are scanned by various input systems of the digital computer system, and are used to determine the operation of the turbine in response to steam inlet valve stimuli. Turbine speed is one such process variable. This single process variable is used within the DEH not only to affect automatic closed loop turbine speed control during startup but also to establish varying protective limits on vibration, eccentricity, acceleration, heat soak periods and even the transfer of control from throttle valves to governor valves. Loss of this crucial turbine speed information to the programmed digital computer of the DEH would disable the automatic turbine speed control and activate the transfer of steam inlet valve control to a degraded backup analog manual system. Therefore, it is evident that the turbine speed measurement is one of the most essential of the monitored process variables and the importance of a speed monitoring system for the generation of a highly reliable and available turbine speed measurement is paramount.
Copending patent application Ser. No. 722,779 entitled "Improved System and Method for Operating a Steam Turbine and Electric Power Generating Plant" filed by Giras and Birnbaum on Apr. 4, 1968, and continued as Ser. No. 124,993 on Mar. 16, 1971, and Ser. No. 319,115 on Dec. 29, 1972 discloses a DEH turbine control system with a programmed digital computer, an overspeed protection controller, and a degraded backup analog manual system, to which reference is made for a more detailed understanding of a DEH turbine control system. A typical overspeed protection controller for a steam turbine generator is more specifically described in U.S. pat. No. 3,643,437. Also, a typical degraded analog backup manual system is more specifically described in U.S. Pat. No. 3,741,246. The latter patent also describes the transfer operation associated with transferring control of steam inlet valves from the programmed digital computer to the degraded backup analog manual system and vice versa. The analog backup system is used to increase the availability of control of the steam turbine. The overspeed protection controller (OPC) is incorporated within the DEH to operate independently of the programmed digital computer to anticipate a possible overspeed condition and protect the turbine plant by rapid closure of the steam inlet and reheat control valves.
Speed information is detected through three speed transducers located in close proximity to a notched surface on the turbine shaft. Each speed transducer is a primary source of speed information for a particular controller or instrument. For example, one speed transducer signal is converted to digital form and coupled to the programmed digital computer. A second speed transducer signal is converted to analog form and coupled to the overspeed protection controller and finally, the third speed transducer signal is coupled to a speed monitoring supervisory instrument where it is converted to an analog form. The OPC and supervisory instrument analog speed channels are also coupled to the programmed digital computer through its analog input system. The speed monitoring system of the digital computer selects one of the three speed readings as the actual turbine speed measurement. In this selection the primary digital speed channel is always given preference. The OPC analog speed reading is chosen as the secondary or backup speed measurement. Only if a malfunction is detected in the digital speed reading will the secondary analog speed reading be selected. The supervisory instrument analog speed reading is used as a reference in the digital computer to determine a malfunction in either of the other two speed readings. Because of the three different conversion methods and interface techniques employed to couple these speed readings to the programmed digital computer, under certain conditions these speed readings will not be of the same value.
It is possible for a number of undesirable effects in turbine operations to occur as a result of the primary-secondary priority selection in combination with unequal speed readings. As one example, should the digital speed channel incur an intermittent malfunction, then an oscillating condition could exist in selecting between the digital and the OPC analog speed channels. The programmed digital computer will respond to the falsely varying actual speed measurement by continually trying to correct steam flow to convert the actual speed measurement to that desired of the power plant operator, thus compounding the problem by producing a disturbing oscillatory valve movement and steam flow. Another example occurs in the transfer from throttle valve to governor valve steam inlet control, which is governed by an actual turbine speed measurement value. Just prior to this transfer speed point, temperature readings are taken to determine the temperature gradient in the steam flow between the throttle valves and governor valves. If the temperature gradient is not within predetermined limits, the transfer will not be permitted and the turbine speed will be maintained at the transfer speed point. Should a malfunction in speed channel occur around this point, the transfer from one speed reading to another could affect a change in actual speed measurement such that a throttle valve to governor valve transfer would be permitted to take place without first checking the temperature gradient criteria.
The OPC as described in the aforementioned references is governed by only one analog speed reading. In this controller, a malfunction of speed channel is determined by checking the speed reading against predetermined high and low physical operational limits. A problem may arise because the predetermined overspeed limit is within this operational range. If the analog speed channel should happen to malfunction and drift between the OPC predetermined limit and the high malfunction limit, an overspeed action could be performed. This is a safe method of operation, but could undesirably initiate an OPC action. This problem condition can be resolved by improving the speed monitoring system of the OPC such that it may perform the same speed monitoring functions as the programmed digital computer.